Conventionally, internal combustion engines having a cylinder head cover are known. For example, Japanese Patent Document JP-B-Hei 7-99087 discloses an internal combustion engine provided with a cylinder head cover formed with: a plurality of ignition plug insertion holes disposed in a straight line in a predetermined direction; a chain case extending in a direction perpendicular to the predetermined direction in which the plurality of ignition plug insertion holes are arranged and spaced a predetermined distance apart from the plurality of ignition plug insertion holes; and an L-shaped gas passage disposed along one longitudinal side and one lateral side of a rectangular region where the plurality of ignition plug insertion holes are disposed. In this internal combustion engine, the L-shaped gas passage is provided to separate oil from blow-by gas (fuel gas containing oil) when blow-by gas with oil mixed therein flows therethrough. For this internal combustion engine example, a fresh air passage for taking in fresh air from the outside of the internal combustion engine and allowing the fresh air to flow therethrough is provided, besides the L-shaped gas passage, on the other longitudinal side of the rectangular region where the plurality of ignition plug insertion holes are disposed.
In the internal combustion engine disclosed in the above example, blow-by gas (fuel gas containing oil) flows through the L-shaped gas passage disposed along one longitudinal side and one lateral side of the rectangular region where the plurality of ignition plug insertion holes are disposed, while the fresh air passage is formed on the other longitudinal side of the rectangular region where the plurality of ignition plug insertion holes are disposed. In other words, only one longitudinal side, but not the other longitudinal side, of the rectangular region of the cylinder head cover where the plurality of ignition plug insertion holes are disposed is used for blow-by gas to flow through, and it is therefore difficult to provide this internal combustion engine with a sufficiently long passage for separating oil from blow-by gas.